defernumbers causes problem in very specific case

[moewew]

Via https://tex.stackexchange.com/q/366106/35864

Unfortunately, I could not reduce the MWe further, but even though it uses biblatex-chicago, the problem is with the defernumbers option.

\documentclass{article}
\usepackage[defernumbers=true,annotation,backend=biber,notes]{biblatex-chicago}

\DeclareBibliographyCategory{cited}
\AtEveryCitekey{\addtocategory{cited}{\thefield{entrykey}}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@collection{author:main,
  location = {Somewhere},
  title = {Collection Title},
  publisher = {Publisher},
  editor = {The Editor},
  date = {9999}
}

@incollection{author:item,
  author = {The Author},
  title = {Item Title},
  pages = {999},
  crossref = {author:main}
}

@book{uncited:book,
  location = {Elsewhere},
  title = {Book Title},
  publisher = {Other Publisher},
  author = {Uncited Author},
  date = {9999}
}
\end{filecontents}

\addbibresource{\jobname.bib}

\nocite{*}

\begin{document}
Some text \autocite{author:item}.

\printbibliography[category=cited]% default title for `article` class: "References"
\printbibliography[title={Further Reading},notcategory=cited]
\end{document}

If compiled with

1. LaTeX
2. Biber
3. LaTeX
4. LaTeX

fails on the last step with

! Undefined control sequence.
<argument> ...xt@context }{\abx@field@localnumber

A few tests suggest that it has to do with https://github.com/plk/biblatex/blob/dev/tex/latex/biblatex/biblatex.sty#L7055 where is not defined when the thing is supposed to be written.

[plk]

Hmm, it seems only impact biblatex-chicago and that's fairly complex to diagnose. It should be fairly obvious - some option which gets into the code paths where localnumber isn't defined like omitnumbers or something ...

[moewew]

I suppose it has to do with the fact that biblatex-chicago calls a cite command in the bibliography here. Many styles don't do that.

I'm not sure what happens, but we get to the point where we try to write \abx@field@localnumber to the aux even if it is undefined. I could fix the problem by adding a guard in https://github.com/plk/biblatex/blob/dev/tex/latex/biblatex/biblatex.sty#L7055 that checks if \abx@field@localnumber is defined. But I'm not sure if that is really what should be done, and if I'm treating the symptom or the real illness there.

[plk]

It's probably ok - I thought this was already dealt with as a year or so ago I had a similar issue with some other style doing similar things.